A heterojunction unit of a barrier film and a GaN (gallium nitride) film formed of InxAlyGa1-x-yN (indium aluminum gallium nitride; 0≦x≦1, 0≦y≦1, 0≦x+y≦1) which is a nitride semiconductor has a high breakdown electric field and a high sheet carrier concentration as compared with a diode constituted by a p-n junction using silicon or the like. Thus, it is proposed that, by using a diode (Schottky barrier diode) for such a heterojunction unit, the breakdown voltage is improved as compared with the diode composed of the p-n junction such as silicon so that a diode performance such as reduction of on-resistance is improved.
For example, Japanese Patent Application Laid-Open Publication No. 2009-117485 (Patent Document 1) discloses a Schottky barrier diode, which includes: a stack structure having a heterojunction unit in which a GaN layer and an AlGaN (aluminum gallium nitride) layer are stacked; a Schottky electrode that forms a Schottky barrier contact with the heterojunction unit and is formed on a first end of this stack structure; and an ohmic electrode that forms an ohmic contact with the heterojunction unit and is formed on a second end of this stack structure. However, Patent Document 1 does not mention forming of an insulating film such as an oxide film between the heterojunction unit and the Schottky electrode.